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NASA | De-Twinkling the Stars

Learn how adaptive optics help produce clearer images from telescopes. Discover how Earth's atmosphere refracts the light from stars and causes them to twinkle when we look at them from Earth's surface. Explore ways to reduce atmospheric distortion and investigate how the Keck Observatory's deformable mirror can compensate for the distortion.

Earth's atmosphere is turbulent—there are layers and pockets of air with different temperatures, compositions, and densities. This is caused by several factors, such as Earth's rotation, topography, convection currents, and particulate matter. Each region of the atmosphere has a different index of refraction—a measure of how much the speed of light is slowed in that medium. When light crosses a boundary between any two materials with different indices of refraction, it changes direction, or refracts. How much light refracts as it travels through the atmosphere depends on the qualities of the regions it travels through.

The original wave front of light from a source such as a star should be smooth and spherical; however, astronomical objects are so far away that their wave fronts actually appear flat by the time they reach Earth. The turbulence in Earth's atmosphere distorts the wave front as it passes through each region: instead of being smooth, like a flat sheet of paper, it becomes distorted, like a wrinkled sheet of paper. This effect is what makes stars appear to twinkle and why telescopes on Earth's surface are limited in how clearly they can capture astronomical images. However, a technology called adaptive optics (AO) can help produce clearer images by correcting for the distortion.

In an adaptive optics system, a deformable mirror can be adjusted to match the shape of a wave front (and basically undistort the image). The AO system determines the distortion of the wave front by using a reference object, such as a star. A Shack-Hartmann wave front sensor is commonly used. This is composed of an array of small lenses; each focuses part of the light from the reference object's wave front onto a sensor, creating multiple spots of light. A flat wave front produces an even distribution of spots, and a distorted wave front produces an irregular distribution of spots with displacements that can be measured. The distortion for the light going through each lens can be calculated from the difference in position of the focused spot of light on the sensor with respect to reference values. The array of lenses allows for simultaneous measurements across the entire wave front.

Changing the shape of the telescope's mirror compensates for the distortion to the wave front. There are a variety of types of deformable mirrors. The Keck telescopes are composed of segmented flat mirrors that can each be controlled in three dimensions: piston (up and down), tip, and tilt. Other possibilities include a single mirror that has actuators (mechanical devices that convert energy into motion) that bend a thin, flexible surface; a mirror made with layers of two or more different materials—one with the ability to change shape in response to electric current; or a liquid mirror made with a ferrofluid (a fluid that contains iron) so that its surface can be controlled by a magnetic field.

Once the wave front is corrected for a reference object, other objects in the same field of view will also be corrected for (because the light has passed through approximately the same turbulence). However, not all astronomical objects are located next to a bright star that is suitable as a reference object. To solve this problem, astronomers use lasers (at a wavelength of 589 nm) that excite the sodium atoms in the naturally occurring sodium layer in Earth's atmosphere, causing a glow that serves as an artificial reference star for the AO system.

This activity demonstrates a simplified version of how an adaptive optics system works. It shows just a few mirrors moving in two dimensions and affects only the profile of a wave. In a real AO system, the mirror would be adjustable in three dimensions and correct for the entire wave.